A faithful reader provides the secret:
If the root is not a complete word, add -ible.
aud + ible = audible
If the root is a complete word, add -able.
accept + able = acceptable
If the root is a complete word ending in -e, drop the final -e and add -able.
excuse - e+ able = excusable
And, this being English, we must note that there are exceptions:
The joy this gave me, to know the rule at last: proof, as if more were needed, that I'm peculiar and needed long ago to get a real life.